As is generally known in the art, cable-stayed bridges support the deck of a main span using inclined cables installed on a main tower. Due to a possibility of increasing the main span, the cable-stayed bridges have recently been constructed for wide rivers and seas.
The cable-stayed bridges are constructed to sequentially install deck segments on opposite sides of a main tower to form a main span and a side span, wherein the deck segments of the main and side spans are interconnected using cables.
Thus, the deck segments interconnected on the opposite sides are subjected to a compressive stress in a horizontal direction.
In detail, as in FIG. 1a, cables 1 interconnect deck segments 2 on opposite sides of each main tower 3 (at main and side spans). As such, among the components of force applied to each cable 1, the horizontal component of force F2 acts on the deck segment 2 as a compressive stress, whereas the vertical component of force F1 acts upwards.
The compressive stress is maximal at a point M where each main tower 3 is installed, and is zero at the middle point C of a main span. This is because, as the deck segments 2 begin to be installed from the main tower, the applied compressive stress is accumulated and increased on the deck segments 2.
Thus, the maximum compressive stress applied to the deck segments 2 is increased in proportion to the main span L, i.e. a distance between the main towers 3.
FIG. 1b is a graph showing a relationship between the maximum compressive stress and the main span L. This graph is obtained on the assumption that a cross-sectional area of each deck segment 2 of the main span is constant.
For example, it can be found that the maximum compressive stress is 160 MPa when the main span L is 1000 m, but it is increased to 500 MPa when the main span L is 2000 m.
In order to cope with the increase of the compressive stress, the deck segments 2 must be formed of high-strength steel, or be increased in cross-sectional area.
Among these methods, the former can cope with the increase of the compressive stress to some extent. However, if the main span L approaches 2000 m, the mere use of the high-strength steel fails to sufficiently resist the applied compressive stress.
Furthermore, due to the applied compressive stress, local buckling may be generated from the deck segments 2 formed of high-strength steel. To prevent the local budding, stiffeners (e.g. longitudinal and transverse ribs) are densely disposed inside the deck segment 2. Thus, a dead load of the deck segment 2 is increased. For this reason, the cables 1 and the main towers 3 are designed on the basis of the deck segments 2 whose dead load is increased, which leads to an increase in size.
Further, the maximum compressive stress occurs at the place M where each main tower 3 is installed. As in FIG. 1a, since the magnitude of the compressive stress is gradually reduced around each main tower 3, a range B where the stiffeners for preventing the local buckling are required to be installed becomes relatively wide. Thus, a working process of the deck segments 2 accompanied with the installation of the stiffeners becomes complicated.
Meanwhile, Table 1 below compares amounts of steel required to build an earth-anchored suspension bridge and a self-anchored cable-stayed bridge as in FIG. 1c according to the main span L.
(Generally, a bridge called a “cable-stayed bridge” is used herein to refer to a “self-anchored cable-stayed bridge” for comparison with the “partially and fully earth-anchored cable-stayed bridge” according to the present invention. It should be considered in Table 1 that a unit cost of steel required for the cables of the self-anchored cable-stayed bridge is higher than that of steel required for the cables of the earth-anchored suspension bridge, and that a unit cost of steel required for the main span and the main towers of the earth-anchored suspension bridge is higher than that of steel required for the main span and the main towers of the self-anchored cable-stayed bridge.)
TABLE 1SteelforSteel for main spanTypecablesand main towersMain spanEarth-anchored suspension 7,500 t23,000 t(L): 1000 mbridgeSelf-anchored cable-stayed 3,900 t25,000 tbridgeMain spanEarth-anchored suspension36,000 t55,000 t(L): 2000 mbridgeSelf-anchored cable-stayed19,000 t94,000 tbridge
As shown in Table 1, it can be found that the self-anchored cable-stayed bridge having the main span L between 1500 m and 2000 m does not draw economical attraction compared to the earth-anchored suspension bridge.
This is because, as the main span L increases, the compressive stress acting on each deck segment of the main span increases, and thus the cross-sectional area of each deck segment of the main span must be increased. As a result, an amount of required materials (steel) is also increased.
Recently, many long span bridges have been constructed to cross wide rivers or seas. If the magnitude of the compressive stress acting on the cross section of each deck segment of the main span can be reduced even when the main span is increased at the self-anchored cable-stayed bridge constructed as the long span bridge, it can be seen that this requirement is essential regarding economical construction of the self-anchored cable-stayed bridge.
For this reason, various studies have been made of a method capable of economically constructing the self-anchored cable-stayed bridge, i.e. reducing the magnitude of the compressive stress acting on the cross section of each deck segment. Among the studies, FIG. 1d shows a method proposed in 2006 by Prof Gimsing.
In detail, two main towers 3 are constructed. Then, one end of a tension cable 6 is connected to an anchorage 5 installed on the side of a side span of each main tower via the top of the main tower, and the other end of the tension cable 6 extends from each main tower toward the main span and is connected to central deck segments 4 located at a middle part of the main span.
Thus, it can be found that a tensile stress T is generated from the central deck segments 4 by the tension cable 6, and that a compressive stress C is generated by the cables from the compressive deck segments, which are connected with the tensile deck segments 4 and are installed in sections L2 from which the tensile deck segments 4 are excluded, without generating an excessive compressive stress as in the related art. As a result, it can be found that it is possible to reduce the cross-sectional area of each deck segment to economically construct the self-anchored cable-stayed bridge.
Here, a process of installing the tension cable 6 and the central deck segments 4 will be described below.
First, as in FIG. 1e, a first main tower 3 and a second main tower 3′ are installed apart from a predetermined distance L, and first and second anchorages 5 and 5′ are installed.
Here, the first anchorage 5 is a reinforced concrete structure installed on the ground G located outside the side span apart from the first main tower 3.
Further, the second anchorage 5′ is a reinforced concrete structure installed on the ground G located outside the side span apart from the second main tower 3′
Continuously, a temporary ropeway 7 is installed to connect the first and second main towers 3 and 3′, and tension cables 6 are installed using the temporary ropeway 7.
To install the tension cables 6, a moving device 8 traveling along the temporary ropeway 7 may be used. The tension cables 6 are moved to the middle between the first and second main towers 3 and 3′ using the moving device 8, and then are hinged to opposite sides of a connection member 9 that is detachably mounted on a lower portion of the moving device 8.
Thus, as in FIG. 1f, the tension cables 6 can be connected by the connection member 9 separated from the moving device 8, thereby sagging in a downward direction.
Meanwhile, the opposite ends of the connected tension cable 6 are anchored to the first and second anchorages 5 and 5′.
After the tension cables 6 are hinged to the opposite sides of the connection member 9, a deck segment 21 is moved to the middle of the main span using the moving device 8 as in FIG. 1g. The tension cables 6 coupled to the connection member 9 are connected to the deck segment 21, and then the connection member 9 is removed.
After the deck segment 21 is installed, other deck segments 22 and 23 are installed on opposite sides of the deck segment 21 using the moving device (not shown) as in FIG. 1h. 
Here, the deck segment 22 installed on one side of the deck segment 21 is connected to the first anchorage 5 by the tension cable 6, and the deck segment 23 installed on the other side of the deck segment 21 is connected to the second anchorage 5′ by the tension cable 6. Thus, it can be found that the deck segments 21, 22 and 23 are pulled in opposite directions by the tensile tables 6, and are subjected to a tensile stress T as in FIG. 1i. 
Meanwhile, as in FIG. 1i, deck segments 40 are installed on the opposite sides of the first and second main towers 3 and 3′.
The deck segments 40 on the opposite sides of the first main tower 3 are mutually connected by respective compressive cables 50, and are each subjected to a compressive stress C applied to the first main tower 3 by the horizontal component of force generated from each compressive cable 50.
As in the first main tower 3, the deck segments 40 on the opposite sides of the second main tower 3′ are mutually connected by respective compressive cables 50, and are each subjected to a compressive stress C applied to the second main tower 3′ by the horizontal component of force generated from each compressive cable 50.
Then, the deck segments 21, 22 and 23 are connected with the deck segments 40 on the opposite sides of the first main tower 3, and the deck segments 21, 22 and 23 are connected with the other deck segments 40 on the opposite sides of the second main tower 3′. Thereby, a self-anchored cable-stayed bridge 10 can be finished.
Consequently, the method of constructing the self-anchored cable-stayed bridge has an advantage in that it can generate the tensile stress T at the main span to reduce the compressive stress C acting on the entire self-anchored cable-stayed bridge, and a disadvantage in that it is somewhat complicated and it is not easy to control the tensile stress T at the main span.
Further, as shown in FIG. 1i, windproof cables 60 are installed to restrict positions of the deck segments during construction of the self-anchored cable-stayed bridge, because the deck segments are subjected to vibration and displacement in vertical and horizontal directions by wind.
Typically, a method of installing blocks on an underwater ground below the deck segments, connecting one end of the windproof cable to the block, and connecting the other end of the windproof cable to the deck segment is used.
However, to install the windproof cables 60, the blocks are submerged in the water. This leads to poor constructability. The windproof cables 60 obstruct the passage of ships on the water, i.e. have a possibility of causing safety accidents. In terms of characteristics of the self-anchored cable-stayed bridge, it is for the most part difficult to avoid installing the windproof cables 60. Thus, there is a need for technological development of a method capable of replacing this construction.